1. Field of Invention
The present invention relates to transmission of data streams with time- or spatially dependent correlations, such as speech, audio, image, handwriting, or video data, across a lossy channel or media. More particularly, the present invention relates to a frame erasure concealment algorithm that is based on reestimating gain parameters for a code excited linear prediction (CELP) coder.
2. Description of Related Art
When packets, or frames, of data are transmitted over a communication channel, for example, a wireless link, the Internet, or radio broadcast, some data frames may be corrupted or erased, i.e., by the channel delay, so that they are not available or are altogether lost when the data frames are needed by a receiver. Frame erasure occurs commonly in wireless communications networks or packet networks. Channel impairments of wireless networks can be due to the noise, co-channel and adjacent channel interference, and fading. Frame erasure can be declared when the bit errors are not corrected. Also, frame erasure can result from network congestion and the delayed transmission of some data frames or packets.
Currently, when a frame of data is corrupted, an error concealment algorithm can be employed to provide replacement data to an output device in place of the corrupted data. Such error handling algorithms are particularly useful when the frames are processed in real-time, since an output device will continue to output a signal, for example to loudspeakers in the case of audio, or video monitor in the case of video. The concealment algorithm employed may be trivial, for example, repeating the last output sample or last output frame or data packet in place of the lost frame or packet. Alternatively, the algorithm may be more complex, or non-trivial.
In particular, there are a wide range of frame erasure concealment algorithms embedded in the current standard code excited linear prediction (CELP) coders that are based on extrapolating the speech coding parameters of an erased frame from the parameters of the last good frame. Such a technique is commonly referred to as an extrapolation method.
For example, a receiver using the extrapolation method, upon discovering an erased frame can attenuate an adaptive codebook gain gp and a fixed codebook gain gc by multiplying the gain of a previous frame by predefined attenuation factors. As a result, the speech coding parameters of the erased frame are basically assigned with slightly different or scaled-down values from the previous good frame. However, as described in greater detail below, the reduced gains can cause a fluctuating energy trajectory for the decoded signal and thus degrade the quality of an output signal.